1. Field of the Invention.
This invention relates generally to joint prostheses, and more particularly to a modular bearing component for an artificial joint and method of replacement thereof.
2. Description of the Prior Art.
Numerous artificial joint devices have been developed. In such joints as the shoulder or hip, the artificial or prosthetic joint typically includes a bearing head and bearing socket which are pivotable or rotatable with respect to one another, and each connected to opposing bone structures. From time to time, deterioration, infection or patient growth will necessitate a change in the prosthetic joint structure, such as providing a different sized bearing head or replacing one or more components of the prosthetic joint. For this reason, modular prosthesis systems have also been developed.
Modular prosthesis systems are broadly accepted in the field of total hip replacements, with virtually all major orthopedic manufacturers in the United States and abroad offering femoral stem components with interchangeable bearings secured to the stem component by a self-locking tapered arrangement ("Morse" taper). The basic design for such hip prosthesis systems is essentially identical, and consists of a male shank which is machined on the femoral stem and which has a diameter in the range of 0.500 inches, a length of approximately 0.400 to 0.700 inches, and an approximately three degree included angle taper on the diameter. Bearing components of different diameters are machined with an identical matching female taper and are assembled with the femoral stem by impaction. One example of such an arrangement is shown in Harris U.S. Pat. No. 4,406,023.
Once a bearing component has been impacted onto a femoral stem, it is firmly secured thereto and requires considerable axial opposing forces to cause disassembly. Such modular prosthesis systems provide a number of advantages, including the requirement of lower femoral stem inventories associated with the use of different diameter bearing components for the same stem, and the adjustability of the effective length of the neck portion of a stem by changing the depth of the female taper of the bearing components. In addition, surgical exposure during revision or replacement surgery is improved by the bearing component being removable, and the bearing components themselves are easily changeable during such surgery.
Essentially all of the modular hip prosthesis systems include a device or specific structural feature to facilitate removal of the bearing component from the femoral stem during such surgery. These devices or features provide a means for applying an axial opposing force to the inferior margin of the bearing component (typically adjacent the female taper thereon) and usually involve a wedge or similar device which is driven between the underside of the bearing component and a broad collar and/or similar surface of the femoral stem.
While the use of modular bearing components connected via the Morse taper is widespread with respect to prosthetic hip systems, it has not yet gained acceptance in artificial shoulder joints. The functional anatomy of the shoulder differs greatly from the hip, due principally to the dramatically greater range of motion required for optimal shoulder function. Thus, while the extension of the modularity concept to a shoulder prosthesis (i.e., a prosthetic stem with interchangeable bearing components) seems advantageous, certain physical obstacles must be overcome.
The following United States patents illustrate various schemes for prosthetic shoulder joints:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 3,815,157 Skorecki et al. 6/11/74 3,869,730 Skobel 3/11/75 4,030,143 Elloy et al. 6/21/77 4,040,131 Gristina 8/9/77 4,206,517 Pappas et al. 6/10/80 ______________________________________
Each of these arrangements presents a somewhat complex joint, which necessarily requires the removal of more adjacent bone structure than desired in order to accommodate the prosthesis joint. Amstutz et al. U.S. Pat. No. 4,261,062 shows a shoulder joint prosthesis which is much simpler in design and requires less bone removal, but does not provide a modular bearing component arrangement.
Perhaps the main reason that modularity has not yet gained acceptance in prosthetic shoulder joint schemes is anatomical. The anatomy of the humerus does not include a lengthened neck extending between the humeral head and shaft. Unlike a femoral component, therefore, there is no "neck" on an anatomically designed humeral stem component. The provision for such a neck requires the removal of more bone from the humerus than desired, and in prior art integral humeral stem prostheses, the inferior margin of the bearing is preferably placed in direct proximity with the level of resection of the humerus. A humeral stem component fabricated with a typical small diameter male tapered shank (i.e., 0.500 inches) to accommodate a bearing component with a mating female taper bore would, when assembled, have an identical appearance to a conventionally integral humeral stem prosthesis. When implanted, however, the inferior margin of such a modular humeral bearing component would transfer forces directly to the resected humerus and not necessarily to the humeral stem component. The absence of compressive force maintained between the bearing and stem components would permit separation of those components and distal migration of the stem component, which of course would result in clinical failure. In addition, the close proximity of the inferior margin of the bearing component and humerus (with no exposed shoulder of the humeral stem component therebetween) means removal of the bearing component from frictional engagement with the humeral stem component requires the exertion of an opposing force directly between the inferior margin of the bearing component and the humerus itself, with the potential for damaging the humerus, an obviously undesirable consequence of using a modular humeral bearing.
Three modular shoulder prosthesis systems are known, the "Fenlin Total Shoulder" by Zimmer, Inc., the "Bio-Modular Total Shoulder" by Biomet, Inc., and a third by Interedics Orthopedics. In the first two systems, a proximal disk-shaped flange is provided on the humeral stem component, with a male Morse taper shaft extending upwardly therefrom. In the Intermedics shoulder, the proximal flange is a portion of a disk, which extends anteriorly and posteriorly from the central Morse taper shaft. In all three systems, a solid metal bearing component with a female Morse taper on its marginal side is frictionally mounted onto the male Morse taper shaft. Removal of the bearing component from the humeral stem is accomplished by exerting a prying force between the marginal side of the bearing component and the proximal flange of the humeral stem component. To facilitate such prying, a slight gap is provided between the marginal side of the bearing component and the exposed surface of the flange. In earlier, one-piece humeral bearing implants, efforts have been made to be certain that all exposed margins of the bearing component are as smooth and rounded as possible, not only to effectuate a smooth rotation of the bearing component on the glenoid, but also because the various tendons of the shoulder, particularly the rotator cuff tendons, must ride directly across the surface of the bearing component. In a total shoulder replacement patient, the rotator cuff tendons are often attenuated and weakened, and the discontinuity caused by the gap between the marginal surface of the bearing component and the flat edges of the proximal flange in the "Fenlin," "Bio-Modular," and Intermedics shoulder systems is undesirable, because it may contribute to further weakening of the tendons. In addition, the bearing components for the "Fenlin," "Bio-Modular," and Intermedics shoulder systems are solid metal, and thus relatively heavy, which can be significant in the function of a limb suspended against gravity.